1. Field of the Invention
The invention relates to a method for calibrating a tool center point of tools for industrial robots comprising a calibration apparatus that has at least two light barriers which are angled to one another with a vertex angle α greater than zero in each case and cross one another at a crossing point, exhibiting the steps of:                a) fixing ACTUAL TCP positional coordinates of a DESIRED tool center point of the tool with reference to a tool reference point of an industrial robot, and to a TCP coordinate system referred to the tool center point, and        b) moving the tool with reference to the TCP coordinate system through the light barriers such that the tip of the tool corresponding to the tool center point interrupts the light barriers.        
2. Description of Related Art
In order to approach any desired points within a working space, industrial robots have a number of interconnected arms, a hand flange at the end of the last arm of the mutually concatenated arms, and a tool that is fitted on the hand flange. The tool can be, for example, a gripper, a welding head or the like.
The position and orientation of the hand flange or the tool center point of a tool fitted on the hand flange can be specified in a stationary robot-independent world coordinate system, or in a stationary base coordinate system referred to an anchor point of the industrial robot. By contrast, the description of the position of the degrees of freedom, that is to say axes and the hand orientation, is done in robot coordinates, wherein starting from the principal axis of the robot, that is to say of the base coordinate system, there is defined for each arm an robot axis coordinate system that describes the relative position of each axis with reference to its preceding axis. The relationship of the robot axis coordinate systems of an industrial robot is described by defined coordinate transformations. By prescribing the position and the orientation of the hand flange or of the center point of a tool in the world coordinate system, it is therefore possible by means of coordinate transformation to calculate the robot axis coordinates in order to be able to drive the individual axes of the industrial robot.
The position of a center point of a tool that is fitted on the hand flange of the industrial robot is described by means of so-called TCP positional coordinates. The programming of the industrial robot is performed on the basis of the hand flange and the fixed TCP positional coordinates. The TCP positional coordinates are supplied along with each tool and are known as tool center point (TCP). Just like the robot axis coordinates, the TCP positional coordinates are in each case a vector with six dimensions. The first three coordinates define the position of the tool center point relative to the tool reference point of the industrial robot, that is to say the fastening point on the tool on the hand flange. The other three coordinates define the orientation of the axis of the tool center point relative to the tool reference point.
The center point of the tool can be, for example, the tip of a welding head. The center point of the tool can be moved precisely only once the TCP positional coordinates are exactly known.
However, during operation the center point of the tool can change owing to tool wear, bending etc., and this leads to defective positioning of the center point of the tool.
There is thus a need to calibrate the center point of tools with high precision.
EP 0 417 320 A1 describes a method for calibrating the center point (TCP) of the tool of an industrial robot in the case of which a setting point is fixed on the hand flange of the robot arm, the position of the setting point relative to the hand flange being known. Furthermore, a reference tip is set up in the working space of the industrial robot. In order to calibrate the tool center point, the tip of the tool is placed on the reference tip, and the position and orientation of the tool tip is determined in a base coordinate system. The setting point of the hand flange is then placed on the reference tip, and the position and orientation of the setting point is determined in the coordinate system of the hand flange. In addition, the position and orientation of the reference tip is determined in the reference coordinate system, and a transformation matrix is calculated from three matrices for the purpose of designating the TCP positional coordinates of the tool center point of the tool.
The calibration requires a multistage movement operation as well as coordinate transformations.
U.S. Pat. No. 6,352,354 B1 describes a light point element for generating a light point signal at a tool center point of an industrial robot. It is possible thereby to describe the exact position of the tool during a learning phase.
U.S. Pat. No. 5,929,584 describes a method for calibrating a tool center point of tools with the aid of a calibration block that has vertical and horizontal surfaces. The TCP positional coordinates of the tool center point are calculated by moving the tool from a starting position up to a contact position of the tool on one of the surfaces, and moving the tool back to the starting point and repeating the operation for the other surface. This disadvantageously requires a complicated coordinate transformation from the reference coordinate system via the individual robot coordinates up to the hand flange, in order to determine the TCP positional coordinates in the TCP coordinate system from the reference coordinates.
DE 37 24 656 T2 describes a multidimensional measuring machine with fork light barriers which meet at a crossing point. Because of the plurality of light barriers, which all lie in the same plane, it is possible to scan the tool in a contactless fashion from different directions but in the same scanning plane.
U.S. Pat. No. 5,177,563 discloses a fork light barrier for calibrating a robot arm in which the tool tip is moved until the latter lies at the crossing point of the light barriers and two light barriers are triggered simultaneously. The TCP positional coordinates at this crossing point are compared with DESIRED TCP coordinates and a deviation is determined therefrom. However, searching for the crossing point is wearisome.